Immunological measurement methods are being used as methods for measuring components in specimens. Immunological measurement methods include many methods such as the RIA method (radioimmunoassay), EIA method (enzyme immunoassay), CLIA method (chemiluminescent immunoassay), CLEIA (chemiluminescent enzyme immunoassay), LA method (latex agglutination method), TIA method (turbidimetric immunoassay), and immunochromatography method. In these assays, when the measurement is carried out by immunological techniques, the antigen-antibody reaction between a component (or antibody) in a specimen and an antibody (or antigen) against it is utilized. Furthermore, in these immunological assays, a calibration curve (standard curve) is made in advance by plotting onto a graph the relationship between numerical values (absorbances) obtained by measuring a standard material having known concentrations and their respective measured values (concentrations), and the measured values of the component of interest in the specimen is obtained. Recombinant antigens which can be prepared in large amounts and for which raw materials are readily available are commonly used for the standard material to be used in these assays.
However, the immunoreactivity of a recombinant antigen does not necessarily match that of the native antigen, which is the component in the specimen, and the reactivity may also differ depending on the buffer and additives used during the assay. In particular, when the reaction temperature during the antigen-antibody reaction changes, the difference in reactivity of the two become prominent, and this causes the problem of producing variations in measured values due to temperature (see Patent Documents 1 and 2).
MxA protein is a protein in the series of proteins induced by type I interferon (interferon α/β), has a molecular weight of 78 kDa, belongs to the Dynamin superfamily, has GTPase activity, and is expressed in the cytoplasm of leukocytes, particularly mononuclear cells. Regarding its function, it is known to have an antiviral effect due to inhibition of virus proliferation, and it is said to be involved in the establishment of antiviral conditions of an organism in the early stage of viral infection (see Non-Patent Documents 1 to 4).
The MxA protein found in several animal species has a characteristic amino acid sequence in its amino acid terminus. The N-terminal G domain is a part necessary for antiviral action and activity as GTPase, and the C-terminal region is abundant in α helical structures and has a leucine zipper structure. These two parts have been reported to react with each other intramolecularly, or cause self aggregation by binding with each other intermolecularly (see Non-Patent Document 5).